Vegetation - a "green" eye for SPOT

31.3.98. First image obtained from
data supplied by Vegetation.

How can we hope to understand climate change, analyse the relationship
between man and his environment, monitor risk areas, follow the progress
of a forest fire, pinpoint a natural disaster, forecast levels of
agricultural production? Simple: by observing the planet from above
and by processing data transmitted by satellite. As the first system
designed specifically to observe the earth's plant cover, Vegetation
offers new prospects for the study, preservation and management of
the plant biosphere. Following its launch on board the SPOT 4 satellite
with the aim of delivering "standard products" and with wide-ranging
support from the EU, Vegetation has reaffirmed the importance of European
space expertise.

On 24 March 1998 the SPOT satellite, with the
Vegetation instrument on board, was launched from Kourou (Guyana).
A week later, Vegetation was sending back its first images. This instrument
has been specifically constructed for monitoring the plant world (in
the past a global view of the earth's forests and crops was possible
only by means of observation systems designed for meteorologists)
and is the result of a European project drawing on the combined effort
of four partners with 46% of the funding (51
million) being provided by the European Union.

"The launching of such a programme has required a level of financial
investment, research input, and pooling of industrial infrastructures
and knowhow that could never have been fully implemented without European
support," explains Michel Schouppe, a member of the scientific staff
in the European Commission's Research DG. "Vegetation is part of a
dynamic package of measures launched by the Commission with the aim
of revitalising the earth observation market through the development
of operational applications which meet the needs of users."

On a global scale
The actual design of Vegetation is dictated by the desire to deliver
"image products" that meet the requirements of the numerous players
involved in the accumulation of knowledge on, and the sustainable
management of, our plant heritage (1). "Vegetation's
specifications were devised in line with the requirements of the
basic research teams working on the functioning of major ecosystems,
changes in the environment and the impact of human activities, and
also in line with much more pragmatic applications - such as the
control of forest fires or the forecasting of agricultural production,"
explains Gilbert Saint, who is responsible for the Vegetation mission
at the CNES. To this end, and in order to monitor developments on
a day-to-day basis, these specialists need high-quality images and
a continuous flow of comparable data that are rapidly accessible
from any point on the planet.

Low and high-resolution
coupling As an on-board component on SPOT, Vegetation has two strings
to its bow: its own technology and the data provided by the other
instrumentation on board the same satellite. Each day this satellite
orbits the earth 14 times, passing directly over the same given
point every 26 days. The conventional, high-resolution images provided
by SPOT sample the ground across a 60 km wide band, although the
photographs obtained from a scan of this nature are inevitably characterised
by a significant time lag.

Observing the evolutionary cycle of plant life only makes sense
if it is done on a sufficiently repetitive scale. This is why Vegetation's
four low-resolution cameras have been designed to provide a "swath"
on the ground of 2250 km. In a 24-hour period Vegetation is capable
of sending back images of 90% of the equatorial zones, whereby the
most extensive zones are overflown and the remaining 10% are automatically
scanned the following day. These images, moreover, are of a remarkable
precision, with very little distortion at the edges. Since the positioning
of each pixel is also extremely precise - 300 metres dispersion
over a year - the analysis of a series of measurements at a given
geographical point, taken over a certain period of time, provides
directly comparable data.

Vegetation has also been equipped with four spectral bands. Infrared
and blue have been added to the two conventional bands (red and
near-infrared) which are indispensable to meteorologists. The infrared,
which is sensitive to the water content of soils and plants, is
suitable for detecting humidity and cover structure. The blue allows
better atmospheric corrections - and, incidentally, certain oceanic
applications - while at the same time opening up possibilities for
soil mapping and the monitoring of soil degradation.

Lastly, the ability to couple images taken simultaneously from
the same platform - those from Vegetation, where each pixel represents
1km2, and those from the other instruments capable of resolving
details to 10m to 20m - makes it possible to refine the analysis.
"A zoom effect with high resolution and the same spectral bands
makes it possible to provide a better description of virtually every
pixel of Vegetation. We are therefore in a position to understand
why every measurement evolves over time as a function of soil occupation
diversity," explains Gilbert Saint.

Four cameras, with a ground swath of 2250
km, will enable images of 90% of the equatorial zones to be
retrieved in 24 hours.

From space to earth

However, this instrument owes its efficiency to important earth-based
components for the reception and preparation of the products, viz.
the Vegetation Programming Centre in Toulouse (France), the data
receiving station in Kiruna (Sweden) and the Vegetation Image Processing
Centre (CTIV) in Mol (Belgium). These facilities are responsible
for the archiving and systematic processing of all the earth's plant
cover. With the help of daily updates it is possible to prepare
summaries covering each decade, and the data can be processed and
delivered to users at exceptionally short notice. "For the Commission,"
Michel Schouppe points out, "Vegetation represents an independent
source of information likely to prove useful in the discharge of
its specific tasks - whether in the area of support for agricultural
production and restructuring, forest management, environmental monitoring,
regional development, preparation of data required for drawing up
international conventions, or the launching of international emergency
food aid operations."

Given that satellites of the SPOT variety have a service life of
about five years, a second research project, Vegetation 2, involving
a large number of manufacturers, will ensure that the needs of the
users of satellite data are targeted even more specifically.

(1) These needs have been identified by an
international users' committee including representatives of international
organisations - such as the FAO - and of international programmes
- such as the International Geosphere Biosphere Programme.